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Posted: Apr 22, 2016

Oscillating currents point to practical application for topological insulators

(Nanowerk News) Scientists studying an exotic material have found a potential application for its unusual properties, a discovery that could improve devices found in most digital electronics.

Under the right conditions the material, a compound called samarium hexaboride, is a topological insulator—something that conducts electricity on its surface but not through its interior. The first examples of topological insulators were only recently created in the lab, and their discovery has sparked a great deal of theoretical and experimental interest.

Now, a team of physicists at JQI and the University of California, Irvine, may have found a use for tiny crystals of samarium hexaboride. When pumped with a small but constant electric current and cooled to near absolute zero, the crystals can produce a current that oscillates. The frequency of that oscillation can be tuned by changing the amount of pump current or the crystal size.

A scanning electron microscope picture of platinum wires on a samarium hexaboride crystal, which can produce an electric current that flips millions of times a second. (Image: A. Stern/UCI)

This behavior—which mimics that of components in modern cell phones and computers in a much smaller package—is due to a complex interaction between currents carried by protected electronic states on the crystal’s surface and the temperature changes they provoke in the crystal’s interior.

"It’s not often that devices rely on such sophisticated ingredients," says JQI Fellow Victor Galitski, who partnered with Dmitry Efimkin, a postdoctoral researcher in the Condensed Matter Theory Center at the University of Maryland, as well as experimentalists at UCI on the new work. Galitski and his colleagues published the results April 20 in Physical Review Letters ("Radio Frequency Tunable Oscillator Device Based on a SmB6 Microcrystal").

The work is the latest development in more than four decades of research on samarium hexaboride (link is external), which is known for its unusual properties at low temperatures. At high temperatures, the material acts like an ordinary metal, but as it is cooled down, its resistance to electrical currents increases. Unlike most materials, however, the resistance doesn’t keep increasing. Rather, it plateaus, even as the temperature continues to drop.

In 2010, Galitski and several colleagues hypothesized that this mysterious behavior arose because samarium hexaboride is a particular kind of material, which they dubbed a topological Kondo insulator. They predicted that such a material would exhibit a transition from a conventional metal at high temperatures to a topological insulator when cooled down. Physicists raced to verify the prediction by trying to observe signature surface currents. Along the way a group at UCI noticed that the material could also produce an oscillating current ("Limit Cycle and Anomalous Capacitance in the Kondo Insulator SmB6").

"That was a completely accidental discovery," says Jing Xia, an associate professor of physics at the University of California, Irvine, and a co-author of the new paper. "We had no clue what was going on."

The frequency of those early oscillating currents was too low to be useful in modern devices, so Xia and Galitski began parallel investigations, with Xia trying to scale down the crystal size to create faster oscillations and Galitski trying to explain the origin of the fluctuating current.

The new paper is the culmination of their efforts. The crystals are now much smaller—just 100 microns long compared to sizes of millimeters in the previous work. This leads to current oscillations that occur tens of millions of times a second, frequencies which can drive radio transmitters or the clocks found in computer chips.

The small size also created headaches for Alex Stern, a graduate student at UCI and lead author of the paper, who painstakingly connected miniscule platinum wires to the tiny crystals in order to make the measurements. "It was really just getting lucky after doing it several times," Stern says. "And once you get one wire on, the hardest thing was to get the second one on right next to it."

Meanwhile, Galitski and Efimkin worked on developing a model to explain the origin of the current oscillations. In the paper, the researchers derive equations that describe how current-carrying charges are transported through the material. Depending on experimental parameters, the charges either flow steadily or fluctuate, the latter corresponding to an oscillating current. Those oscillations appear for only a narrow range of parameters, and the researchers say they were lucky that their sample fall into this regime.

Despite the technical complexity of the theory, Galitski says that the physical mechanism that drives these fluctuations is simple. The essential steps are shown in the graphic above. As a current begins to flow on the crystal’s surface, a high resistance causes some of the current’s energy to heat the sample’s interior. This causes the electrical resistance of the interior to decrease slightly, momentarily allowing more current to flow. But the decreased resistance also dissipates less energy, so the interior cools back down and the original resistance is restored. This process repeats itself many times a second to produce the fluctuating current seen in experiments.

The authors suspect that smaller crystals could produce even faster oscillations, on the order of a trillion per second. The team says that this might happen for crystals that are 10 times smaller than the 100-micron long crystals they are using now. They add that there is no fundamental reason that similar devices that support rapid oscillations cannot be engineered to operate at room temperature.